This invention relates to downhole tools and devices used in oil and gas wells, and more particularly to a method for running downhole tools and devices utilizing coiled continuous tubing into open well bores or well bores having casings.
The use of coiled tubing and coiled tubing equipment to perform many tasks that were conventionally performed by jointed tubular steel piping is well known in the art. Such tasks include the running, or conveying, of downhole well logging tools such as downhole tools having visual and/or acoustic apparatus contained therein by way of coiled tubing, whether it be in vertical, deviated, or horizontal wellbores, or whether the wellbore be open or have casing therein.
Representative prior art patents describing such tasks being performed with coiled tubing include U.S. Pat. No. 4,938,060--Sizer et al., which describes a system and method for visually and/or acoustically inspecting a well bore, and U.S. Pat. No. 5,180,014--Cox, which describes the use of coiled tubing to deploy a submersible electric pump downhole. Both of these patents are specifically incorporated herein as references. Representative prior patents disclosing the use of conventional jointed tubing and coiled continuous tubing specifically for performing logging operations include U.S. Pat. Nos.: 4,685,516--Smith et al.; 4,570,709--Wittrisch; and 3,401,749--Daniel, all of which are also specifically incorporated herein as references.
A shortcoming with the prior art, especially when using conventional jointed tubing for running tools downhole, is the inherent difficulty in running tools downhole in wells that have a relatively high wellhead pressure because means must be provided about the jointed tubing to maintain pressure differentials between the wellbore near the surface and the atmosphere. Thus, there remains a need for a method which allows tools to be run downhole in a convenient manner when the subject well has relatively high pressures at or near the surface where the wellhead is normally located. Such pressures may exceed 2,500 psi (17.3 MPa) and in the past, the well was "killed" or other steps were taken to temporarily reduce the high surface pressures in order for tools to be safely run into that portion of the well of particular interest.
Another shortcoming with the prior art resides in the fact that coiled tubing units used for well logging and/or visual/acoustical inspection have an electrical or an opto-electrical cable installed within a preselected size and length of the coiled tubing that is stored on a reel. Such cables routinely contain electrical leads for powering the tool or device installed on the coiled tubing, and/or contain optical or communication leads for carrying signals generated by the downhole tool, or device, to recordation and monitoring equipment located on the surface. Additionally, the cable may contain electrical control leads, or conductors, which are needed to operate and control various functions and components within the downhole tool or device. Such leads may be of conventional multi-stranded metal conductor wire surrounded by an insulative jacket, or of conventional coaxial cable. Furthermore, the use of fiber-optic glass or plastic leads having various protective shrouds, also referred to as fiber-optic cable are being employed in such downhole cables that are capable of withstanding high pressures. Because the downhole cable, regardless of the type or combination of leads contained therein, is as a practical matter, permanently installed in a given coil of tubing installed in a coiled tubing unit due to the coil of tubing often times can not be removed and replaced in field locations due to the size and weight of the reeled tubing. This results in coiled tubing units being specifically limited to, or dedicated, to operations that can utilize, or at least not be hindered by, the particular electrical or opto-electrical cable that is installed therein. For example, a coiled tubing unit having such a cable installed therein would not be as effective, or perhaps not usable, when used for treatment or stimulation operations because of the obstructing nature of the cable being present within the tubing. The requirement that dedicated coiled tubing units be acquired and maintained results in an economical disadvantage to coiled tubing operators, especially in geographically large or remote areas where such coiled tubing units having an appropriate cable therein are in very limited supply. In such situations, logging and/or inspection jobs must be anticipated and planned several days or weeks in advance to allow for transportation of the required coiled tubing unit having an appropriate cable therein.